Airborne sunphotometer measurements are used to evaluate retrievals of
extinction aerosol optical depth (AOD) from spatially coincident and
temporally near-coincident measurements by the Ozone Monitoring Instrument
(OMI) aboard the Aura satellite during the March 2006 Megacity
Initiative-Local And Global Research Observations/Phase B of the
Intercontinental Chemical Transport Experiment (MILAGRO/INTEX-B). The
14-channel NASA Ames Airborne Tracking Sunphotometer (AATS) flew on nine
missions over the Gulf of Mexico and four in or near the Mexico City area.
Retrievals of AOD from near-coincident AATS and OMI measurements are
compared for three flights over the Gulf of Mexico for flight segments when
the aircraft flew at altitudes 60–70 m above sea level, and for one flight
over the Mexico City area where the aircraft was restricted to altitudes
~320–800 m above ground level over the rural area and ~550–750 m
over the city. OMI-measured top of atmosphere (TOA) reflectances are
routinely inverted to yield aerosol products such as AOD and aerosol
absorption optical depth (AAOD) using two different retrieval algorithms: a
near-UV (OMAERUV) and a multiwavelength (OMAERO) technique. This study uses
the archived Collection 3 data products from both algorithms. In particular,
AATS and OMI AOD comparisons are presented for AATS data acquired in 20
OMAERUV retrieval pixels (15 over water) and 19 OMAERO pixels (also 15 over
water). At least four pixels for one of the over-water coincidences and all
pixels for the over-land case were cloud-free. Coincident AOD retrievals
from 17 pixels of the Moderate Resolution Imaging Spectroradiometer (MODIS)
aboard Aqua are available for two of the over-water flights and are shown to
agree with AATS AODs to within root mean square (RMS) differences of
0.00–0.06, depending on wavelength. Near-coincident ground-based AOD
measurements from ground-based sun/sky radiometers operated as part of the
Aerosol Robotic Network (AERONET) at three sites in and near Mexico City are
also shown and are generally consistent with the AATS AODs (which exclude
any AOD below the aircraft) both in magnitude and spectral dependence. The
OMAERUV algorithm retrieves AODs corresponding to a non-absorbing aerosol
model for all three over-water comparisons whereas the OMAERO algorithm
retrieves best-fit AODs corresponding to an absorbing biomass-burning
aerosol model for two of the three over-water cases. For the four cloud-free
pixels in one over-water coincidence (10 March), the OMAERUV retrievals
underestimate the AATS AODs by ~0.20, which exceeds the expected
retrieval uncertainty, but retrieved AODs agree with AATS values within
uncertainties for the other two over-water events. When OMAERO retrieves
AODs corresponding to a biomass-burning aerosol over water, the values
significantly overestimate the AATS AODs (by up to 0.55). For the Mexico
City coincidence, comparisons are presented for a non-urban region
~50–70 km northeast of the city and for a site near the center of the city.
OMAERUV retrievals are consistent with AERONET AOD magnitudes for the
non-urban site, but are nearly double the AATS and AERONET AODs (with
differences of up to 0.29) in the center of the city. Corresponding OMAERO
retrievals exceed the AATS and/or AERONET AODs by factors of 3 to 10.